The present invention relates to organic aerogels, particularly to carbon aerogels, and more particular to a phenolic-furfural aerogels and to methods of preparing same,
Aerogels are a unique class of ultra fine cell size, low density, open-cell foams, Aerogels have continuous porosity and a microstructure composed of interconnected colloidal-like particles or polymeric chains with characteristic diameters of 100 angstroms, The microstructure imparts high surface areas to aerogels, for example, from about 350 m.sup.2 /g to about 1000 m.sup.2 /g.
Organic aerogels from the sol-gel polymerization of resorcinol-formaldehyde have been developed, as disclosed in U.S. Pat. No. 4,873,218 issued Oct. 10, 1989 and U.S. Pat. No. 4,997,804 issued Mar. 5, 1991 to R. W. Pekala. Also, organic aerogels from the aqueous, sol-gel polymerization of melamine with formaldehyde has been developed as disclosed in U.S. Pat. No. 5,086,085 issued Feb. 4, 1992 to R. W. Pekala.
Carbon aerogels are formed by pyrolyzing resorcinol-formaldehyde aerogels, for example, in an inert atmosphere, and the carbon aerogels may be doped or impregnated with selected materials to increase the electrical conductivity thereof, and provide improved properties over prior developed microcellular carbon form, exemplified by U.S. Pat. No. 4,806,290 issued Feb. 21, 1989 to R. W. Hopper et al.
More recently, carbon aerogel electrodes have been derived from the pyrolysis of resorcinol-formaldehyde and related polymers, which are of intermediate to high density (0.1 to 1.2 g/cc) electrically conductive and have high surface areas (400 to 1000 m.sup.2 /g), and have particular application in energy storage devices, such as double layer capacitors, known as supercapacitors, having capacitances on the order of several tens of farad per gram of electrode. Such carbon foam electrodes are disclosed in U.S. Pat. No. 5,260,855 issued Nov. 9, 1993 to J. L. Kaschmitter et al.
The various prior method of forming the organic aerogels have utilized critical point or supercritical drying procedures, as disclosed in the above-referenced U.S. Patents, and in U.S. Pat. No. 5,252,620 issued Oct. 12, 1993 to J. R. Elliott, Jr. et al. Recently, a new air drying technique has been developed, which is substantially less expensive and less time consuming then the supercritical drying techniques, and is described and claimed in copending U.S. application Ser. No. 08/041,503 filed Apr. 1, 1993, entitled "Method Of Low Pressure And/Or Evaporated Drying Of Aerogel".
Organic aerogels such as resorcinol-formaldehyde (RF) and melamine-formaldehyde (MF) have low thermal conductivities as a result of their ultra fine cell size and low densities. The RF and MF aerogels are formed from a sol-gel polymerization in water, followed by a solvent exchange into acetone, and subsequent supercritical drying from carbon dioxide (T.sub.c =31.degree. C.; P.sub.c =7.4 MPa), as described in detail in above-referenced U.S. Pat. No. 4,873,218 and U.S. Pat. No. 5,086,085, for example. These aerogels have been utilized in a wide variety of applications, such as thermal insulation and ICF targets. In the case of resorcinol-formaldehyde, for example, the aerogel can be pyrolyzed in an inert atmosphere, such as nitrogen, at 600.degree.-3000.degree. C. to form a carbon aerogel. Carbon aerogels or thin film composites formed from them (i.e. non-woven carbon cloth (paper)+carbon aerogel), as disclosed in above-referenced U.S. Pat. No. 5,260,855, for example, are finding applications as electrodes in double layer capacitors (supercapacitors), and other energy storage devices, as well as in fuel cell and deionization systems.
While the prior aerogels have been shown to have a wide variety of applications, either in the aerogel or the carbon aerogel form, it is desirable to develop new organic aerogels which have different densities, cell sizes, and surface areas which can be utilized in the same or in different applications, such as chromatographic packings, water filtration, and ion-exchange, for example. The present invention provides a new type of organic aerogel, a phenolic-furfural aerogel, developed from the reaction of phenolic novolak with furfural, which can also be pyrolyzed in an inert atmosphere to produce carbon aerogels, which has thermal conductivities as low as 0.015 W/m-K, and thus this invention constitutes an advance in organic aerogels.